1. Field of the Invention
The present invention relates to a seam welding method and a seam welding apparatus for welding a workpiece while the workpiece is being delivered between a pair of electrode rolls.
2. Description of the Related Art
Fuel tanks for use on motor vehicles or the like, for example, are manufactured by superposing outer circumferential portions of two members that have been pressed to shape and welding the superposed outer circumferential portions. The welded outer circumferential portions have a two-dimensional or three-dimensional curved shape.
For welding workpieces of the type described above, the present applicant has proposed a method of controlling a seam welding apparatus for holding a workpiece with a workpiece holding jig on the wrist on the effector end of a robot and welding the workpiece while the workpiece is being delivered between a pair of electrode rolls (see Japanese Patent Publication No. 03-068791). According to the proposed control method, the difference between the normal torque acting on the wrist when the actual distance that the workpiece is delivered by the electrode rolls and the distance that the workpiece is fed by the robot are in agreement with each other, and the torque acting on the wrist when the workpiece is actually welded is detected, and the rotational speed of the electrode rolls and the direction of travel of the workpiece are changed depending on the detected difference. In this manner, even if the actual welding track deviates from a desired normal track, the actual welding track can automatically be brought back to the normal track, allowing the workpiece to be seam-welded accurately.
The control method of the above related art requires acquiring a more accurate normal torque (master data) for a more accurate welding operation. For example, a plurality of welding processes are performed on a workpiece on a trial basis while at the same time torques applied respectively during the welding processes are recorded, and the torque applied when the workpiece was welded at a high quality level is regarded as a normal torque.
The above process of acquiring a normal torque is disadvantageous in that since the workpiece is welded on a trial basis without the actual welding track correction referred to above, it may be difficult to achieve a high quality level which is good enough to obtain a normal torque. In addition, the process is tedious and time-consuming because of the need for a plurality of good-quality samples to be produced for increased normal torque accuracy. If the shape or rigidity of the workpiece or the welding rate is changed, then the normal torque needs to be acquired again.
When a highly accurate normal torque has not been obtained, it is difficult to quickly and accurately determine whether the actual welding track deviates from a desired normal track or not, with the result that the workpiece cannot be welded at a high quality level. Furthermore, since different welding regions have different rigidities and different distances from the workpiece holding jig, the actual deviation of the actual welding track from a desired normal track may be different depending on the different welding regions even though the detected torque value remains the same.